1. Field of the Invention
The invention relates to an apparatus and a method for isostatic pressing with microwave heating. Specifically, the invention relates to an apparatus and a method for isostatically pressing ceramic or powdered metal materials while heating the materials by microwave energy.
2. Description of the Background Art
Developments in material science are producing new materials made from ceramics or powdered metals. These new ceramics and metals have characteristics far superior to conventionally produced ceramics or metals. The characteristics of these new materials can include improved toughness, strength, fracture resistance, and thermal expansion coefficients.
The manufacturer of a workpiece or other object from a ceramic or powdered metal material requires an apparatus and a method for compacting and/or applying heat to the material. Apparatuses and methods for applying heat to ceramic or powdered metal materials have traditionally utilized convection furnaces. Certain ceramic materials have been processed by electronic heat generated from either microwave or induction heating. Apparatuses and methods for applying pressure to ceramic or powdered metal materials have utilized both uniaxial and isostatic pressurizing means. The application uniaxial pressure is performed by mechanical means including hot presses. The application of isostatic pressure is performed by either a cold isostatic press, commonly referred to as "CIP" unit, or by a hot isostatic press, commonly referred to as "HIP" unit.
Convection heating is commonly performed with the use of a resistant heating furnace. Resistant heating furnaces are electrically operated furnaces having elements that resist the electric current. Resistant heating furnaces consume relatively large quantities of energy, time, and manpower, compared to other methods of heating, when used in advanced materials production. Resistant heating furnaces create a "hostile" environment for many ceramic or powdered metal materials. The hostile environment can cause sintering and form fractures in workpieces made from these materials during heating. These fractures particularly occur at the surface of the workpieces.
Sintering and fractures result from the more rapid heating of the outer surface of the workpiece in relation to the interior portions or "bulk" of the workpiece. The outer surface of the workpiece convects heat into the interior portion of the workpiece. This causes uneven heating and a delay before the workpiece reaches a uniform heat.
Electronic heating commonly includes induction, dielectric, and microwave heating. These forms of heating vary in their respective apparatuses for generating heat and in the methods by which heating occurs in the material.
Induction heating induces a current in a material. The current is induced by alternating the current in a coil surrounding the material. The coupling of energy in the material is accomplished by magnetic fields. Induction heating typically rises the temperature at the surface of a part first and is best performed with conductor materials such as steel.
Dielectric heating places the material between electrode plates. The energy loss across the electrode plates causes the material to heat. Dielectric heating usually requires a higher frequency of alternating current than is used with induction heating. Dielectric heating is effective with poor electrical and thermal conductors.
Microwave heating applies electromagnetic energy near the far-infrared region. Microwaves have wave lengths from about thirty centimeters to about one millimeter. Microwave energy is created by a microwave generator which produces the electromagnetic energy. The electromagnetic energy is directed to the material by wave guide transmission, resonant cavities, and antennas. Microwave heating applies an alternating or varying electromagnetic field to the material which converts some of the electromagnetic energy into heat in the dielectric or by dielectric loss. Microwave heating is effective with conductive materials and becomes more effective as the temperature of these materials increases.
Microwave energy couples with certain materials and pass through other "transparent" materials. When microwave energy couples with a material, a dielectric loss occurs in the interaction of microwave energy and the material. This dielectric loss creates heat. Transparent materials can be made to couple with microwave energy by elevating the temperature of the material. Materials are generally grouped into "conductors" and "insulators" depending upon whether the materials are readily coupled or are transparent to microwave energy. Relatively poor conductors are termed "semiconductors."
The use of the terms "conductor" and "insulator" with microwave technology is not always desirable. For example, a pure metal is an electrical conductor, but, except at its surface, reflects microwaves. Water is not considered a desirable electrical conductor, but the polarity of its molecules allows it to couple well with microwaves. For these reasons, and for the purposes of describing this invention, the term "conductor" includes any element that is a microwave coupler. The term "insulator" includes any element that is transparent to microwaves. The application of these terms with certain compositions is relative and depends upon the respective dielectric constants of the two or more materials being compared. Generally, the first element in a material to couple with microwaves is considered to be the conductor for that material. The same element in another composition or at a different concentration may not perform as a coupler for the other composition.
The term "coupler" for purposes of this invention includes a conductor or semiconductor dispersed in a nonconductor material. The coupler first develops high electrical charges on its surfaces. This results in capacitance discharges from particle to partical of conductor or semiconductor causing ionization and very rapid heating of the matrix material immediately surrounding the conductor or semiconductor. The ionization and heating of the matrix material results in the matrix material being able to better couple with the microwave field. The term coupler also includes materials that couple well within a given temperature range which assists in the heating of matrix materials and heating to coupling temperature of matrix materials. A coupler can also be a resistive material or sometimes a semiconductor that couples with the microwave field such that it becomes hot enough to heat matrix materials to coupling temperature or to densification temperature.
Materials having polarized molecules provide better couplers for microwave energy than non-polar materials. A mixture of a polarized material with a non-polarized material can generally be heated by microwave energy by the coupling of the polar material first with the microwave energy to create heat. The heat generated by the microwave coupling of the polar material elevates the temperature of the non-polar material by conduction. The rise in the temperature of the non-polar material improves its coupling with the microwave energy. This phenomenon is explained in a book by David A. Copson, Microwave Heating (Westport, CT) The AVI Publishing Company, Inc., 1975).
The application of pressure to ceramic or powdered metal materials compacts or forges the individual grains of the material together. The compaction of the grains of the material increases the density of the material such that a powdered ceramic or powdered metal composition can be compressed into a solid. The compaction of grains of a powdered material can increase the density of that material up to an absolute or theoretical density.
A material of theoretical density has no fissures or separations between the molecules of the material and eliminates any individual grains of the material. A material at its theoretical density is a "perfect solid." Materials that are not at their theoretical density contain internal cracks, fissures, or voids which, when the material is put under stress, grow and expand. The growth and expansion of cracks, fissures, or voids causes a workpiece made from the material to break or fail under stress. Workpieces best resist breakage or failure under stress when their materials are at or near theoretical density.
Pressure is traditionally applied to materials, such as metals, by hot pressing or forging operations. These operations involve the use of a ram or repeated hammering to compact the grains and molecules of the material into a higher density. Mechanical operations such as hot pressing or forging apply uniaxial pressure to a material. Uniaxial pressure can compact the material in a direction parallel to the application of the pressure, but can cause the grains or molecules of the material to spread in a direction perpendicular to the application of uniaxial pressure. The application of uniaxial pressure by hot pressing or forging, typically, cannot uniformly compact a material to its theoretical density. This is because the pressure cannot be applied equally from all directions to the material at the same time.
Pressure can be applied in all directions or "isostatically" to a material through a fluid medium surrounding the material. Ceramic and powdered metal materials can be isostatically compacted in a cold isostatic press or a hot isostatic press.
A cold isostatic press applies isostatic pressure to a placed inside a mold formed by a rubber bag. The rubber bag is placed within a pressure vessel. A medium such as oil is then pumped at high pressures into the vessel so as to surround the rubber bag and isostatically compact the rubber bag and the material within the rubber bag. Cold isostatic presses typically operate at pressures between about 3,000 and about 60,000 pounds per square inch and temperatures between atmospheric temperature and about 500.degree. C. When elevated temperatures are used, the materials are heating before being placed into the cold isostatic press.
A hot isostatic press applies isostatic pressure to a material formed into a green body or to a material within a mold or "can." The green body or can is placed into the pressure vessel. The pressure vessel contains a convection furnace and is pressurized by a gas medium such as argon gas. The pressure and temperature within the vessel can be selectively controlled to obtain desired results for a particular material. Hot isostatic presses can operate at temperatures up to about 250.degree. C. and pressures up to about 100,000 pounds per square inch. A description of methods for isostatic pressing is provided by Clauer, et al., Hot Isostatic Pressing (Columbus, Ohio: Metals and Ceramics Information Center, 1982).
Hot isostatic presses are desirable for compacting powdered ceramic or powdered metal materials to near their theoretical or "near net" density. Hot isostatic presses can elevate the temperature of a material to its transition temperature between being a solid and a liquid while applying very high pressure isostatically through a gas or fluid medium. The use of a hot isostatic press requires a large amount of energy and considerable time to elevate the interior or hot zone of the vessel to a desired operating temperature. This energy and time consumption is required because hot isostatic presses use side zone furnaces having molybdenum radiation plates through which an electric current passes. The electric current is resisted by the molybdenum radiation plates and generates heat through the fluid medium or gas. The heat is convected through the fluid medium to the workpiece.
A workpiece, being processed by a hot isostatic press, is first heated at its surface and is convected through the workpiece to its interior. Once a workpiece is held at the desired temperature and pressure for the desired amount of time, the workpiece as well as the furnace and interior portion of the vessel must be cooled before the vessel can be opened and the densified workpiece removed. The cooling process is typically performed through a liquid cooling liner wherein heat is transferred to a liquid coolant. It is not uncommon for one complete "cycle" of hot isostatic pressing to last between about four hours and about 24 hours. A cycle involves placing the workpiece in the vessel, elevating the temperature and pressure of the workpiece, and cooling of the workpiece. The time required to conduct a hot isostatic pressing cycle, as well as the special operating procedures required for handling advanced materials and operating equipment at high temperatures and pressures, has inhibited the use of isostatic pressing in high production or assembly line manufacturing.
The use of electronic heating with methods for compacting or densifying powdered ceramics or powdered metals has, also, not been generally successful or readily adaptable to high production or assembly line manufacturing procedures. The reasons for the failure to combine methods for electronic heating with compaction of powdered ceramics or powdered metals is often due to the incompatibility of the various apparatuses required to operate an electronic heat generating means with a pressure applying means. The following documents demonstrate the state of the background art for electronically heating compacted powdered ceramics or powdered metals.
U.S. Pat. No. 4,695,695 to Meek, et al., discloses a mixture for producing fracture-resistant, fiber-reinforced ceramic material by microwave heating. This invention prepares a solid ceramic material that is a mixture of glass, a coupling agent, and a resilient fiber, by enclosing the ceramic mixture with an insulating material. The enclosed ceramic material is then subjected to microwave energy for a sufficient time to bond the ceramic material to the fibers. This method is a sintering method wherein a solid is formed from a glass and fiber composite. No pressure is applied to the material during heating. The absence of a pressure application procedure prevents the treated material from obtaining a near net density. Additionally, this method requires coupling agents, such as oils, glycerol, silicon carbide, water, or sugar, in order to affect sintering by the microwave energy.
An article by Krause, "Microwave Processing of Ceramics: An Interdisciplinary Approach", Oak Ridge National Laboratory REVIEW, 1 (1988):48-51, discloses a method for using microwave energy to heat monolithic ceramics without adding impurities. The method of this disclosure obtains an alumina part or workpiece having a density of near 98 percent of the theoretical density. The method of this disclosure uses a high-frequency microwave gyrotron to produce microwaves at 28GHz. The disclosure indicates that alumina is "difficult to heat at 2.45GHz, but it can be heated easily at 28 GHz." This disclosure recognizes the importance of a uniform microstructure in ceramics in order to avoid fractures, but does not disclose a method for applying pressure to a ceramic material being subjected to the microwave energy. The use of a gyrotron, as applied in this method, can produce an average power of 200kW in a continuous wave at the upper-microwave-frequency regime of 28 to 140 GHz. The use of a gyrotron having these characteristics can be prohibitive to many manufacturing facilities due to the expense of the gyrotron and the expertise required to maintain and operate such a gyrotron.
An article by Swain, "Microwave Sintering of Ceramics", Advanced Materials & Processes Inc. Metal Progress (Sept. 1988), 76-82, discloses additional information regarding the Oak Ridge National Laboratory gyrotron discussed in the article above. This disclosure further amplifies the unexpected results achieved with the use of microwave energy in sintering ceramics. Microwave energy allows sintering and annealing to occur at temperatures up to 2000.degree. C. (3600.degree. F.) in a vacuum, inert, or oxidizing environment. This process creates a "hot zone" only around the workpiece which can be contained in an insulating liner. The insulating liner prevents impurities that can contaminate other furnaces from migrating to the furnace wall. This further allows the particular ceramic load or type of ceramic material to be changed from run to run without having to modify the apparatus such as is required with changing furnace elements that would be reactive with impurities in other furnaces. Additionally, this disclosure states that the lack of a requirement for furnace cooling time permits more rapid turn around between sintering processes. This disclosure indicates that high-purity alumina cannot be heated using radiation at frequencies as low as 2.45 GHz and also does not disclose a method wherein pressure is used to obtain a near net density.
Two articles by Gabriele, "New Oil Drill Bits Made via P/M Densification Tested", from an unknown source and "Ceracon Awarded $500,000 Numa Tool Contract for New Powdered-Metal Densification System", Metalworking News (Oct. 17, 1988), each disclose a process for compacting or densifying powdered metal materials. The exact means for heating the powdered metal is not disclosed by these articles. These articles indicate that the powdered metal material in a pre-densified formation or "green state" is formed by a cold isostatic press. Once formed, the green component is coated with a powdered metal slurry for further densification. Further densification is achieved to the "near-net shaped design" by inserting the green body into a circular open-pot die and completely covering the green body with a heated carbon-based or ceramic granular material known as the pressure transmitting medium. Once the component is immersed in the pressure transmitting medium, a vertical ram press compacts the heating granular material. This process is reported to achieve near net densification and is stated to be more desirable than hot isostatic pressing because the component is held at high temperatures and pressures for a greatly reduced amount of time. This reduced amount of time prevents the breakdown of molecular properties in the material. These disclosures do not involve the utilization of microwave heating.
The industry is lacking an apparatus and a method for isostatically pressing a ceramic powdered metal material that is uniformly heated by microwave energy.